Stabilizing chrome ores of the transvaal type for refractory brick



Patented Oct. 20, 1953 STABIEIZINGEDHROME ORES;

.TRANS.VML.,:.TYPE FOR- REFRAC- Russell Pearce-Heuer, Villanova, Pa.,-assignoc to General Refractories Company, a corporation NozDrawing, Applicationlanuary 12,,1952, Serial No. 2.66344" -9 'Glaims. '1 The present invention relates, to the manufacture of? refractory brick by the use. of chrome ore, which; has not. heretofore. been. suitable: for thispurpos.

The presentapplication. is a. continuationrinpart. of copending; application Serial No.. 210,625, filedyliebruary 12,. 19 .1 for: stabilizing- Chrome Ores zof Transvaal; Type. for PvefractoryaBrick, now: abandoned.

purpose of" the invention is'toruse achrome 'ore having a spinel' phase-inwhich the-moi" ratio ofthe A1203 plus CIzOs content to-the' Mgo'content" exceeds 1175 (afterremoving silica).

A 'further purpose to' stabilize the ratio 0f Rzoacontent, to content, which. develops in such a chrome. spinel. after. heating .to .a. high temperatureiby bringing. the chrome ore, into contact, with magnesia. to permit. absorption. of

magnesia: by the. chrome spinel.

A. furtherpurpose is to: manufacture,- .unfired refractory-brick oi, chrome; ore-containing: spinel asset. forth:v above: in:v order. to; eliminate. the

:natural'. shrinkage. of the unfired refractory by corrective expansion in accordance with the present invention.

' A furtherpurpose is to obtain an improved strength at intermediate temperature of a unfired chromite refractory brick' employing a spinel'p hase as a-bove set forth.

further purpose is to" employ 80 to 20" percent" of; chrome ore; having a 'mol' ratio of A1203 plus CI2O3 to MgO exceeding 1.75' after removing silicawith 20 to 80percent ofMgO; The

chrome ore, desirably to the, extent of" 505 to 80 *percentof" thebri'ck, (preferably GBpercent'),

will'be'employed as" coarse partialesbetween. 6 and 28" mesh per linear inch, although magnesia coarseparticl'esmay permissibly be used, and

the magnesia, desirably to. the extent'oi 20 to 510 percentof the brick, will be employed as fine particles, although chrome fine particles may permissibly be used;

Further purposes appear in the specification and: in the claims.

In the manufactureoi refractory brick, the chrome ores generally employed, with, itheir .trpicai analyses are as, follows:

Cuban Philllpine Turkish Ore Ore Ore v Percent Percent Percent 10:00; 0:11 1 0:20 .3-.,1 2 5,10 5,11 13255; 12:83 '12. 62 28:51 329207- 161.80 0.09 0.17. 0.31 1775.0 19:20 18. 37 5312-23 .32.,90 461.22

10mm 99.38 9&63

"IneSechrome oresv contain. a; princ p l ph having, a. spinel. composition together with associated magnesium ilicate impurities. If such chrome. ore is: subjected to they chemical" action of sulphuric acid, plus. hydrofluoric acid .and finally l'eached with dilute hydrochloric acid, the

silicate phase or silica will be removed.

.The silica removal methodhused' studying the, chromaores takes. advantage. of the easy decomposition. of; hydrous silicates and; oxides by hydrofluoric acid; and. sulphuric acid" in aqueous solutions without; substantial" efi'ect on the chromitespinel. "The silica reacts with the hyldrofiuoric .acid" to. liberate silicon fluoride and leaves. 'theother elements originally combined with.the..'si1ica.to react. with the sulphuric acid to form. sulphathe.v The sulphatesare then dissolved" inld'il'ute hydrochloric. acidtand removed .by, washing by. Idecantation. The chromite spinel, which for all practical purposes is. not afiected by these acids, is dried and heated gently to about/200 to 300 C. to remove any films of'acidadhering to; the surface.

The detailed method used for removing the silica is. as follows:

. Treat about five. grams. of chrome. orethrough 200. mesh per linearinchrin a platinum crucible or, dish with 5 cc. ofdilute-sulphuricacid (con,- vtaining equal volumes of water and concentrated sulphuric acid) followed by l5-to- 20 cc. of 5.01%

hydrofluoric acid incwater- "Heat on. an. electric hot plate, with occasional stirring by means of a platinum wire for a half hour. Pour the contents of the crucible into, a 400 cc. beaker containing about '10000: of-dilute" hydrochloric acid (equal volumes: of; concentrated hydrochloric acid and water). Boil'.for.a few minutes, allow "to settle and z'decant the" supernatant liquid.

Add 100 cc. of hot water, stir and again allow to settle and decant. Repeat this water washing about four to five times. Finally dry the residue and heat gently to 200 to 300 C. to remove any acid films still adhering to the chromite grains. It should be kept in mind that this removal of silica is an analytical method used in the pretesting of the materials, and that it is not intended that silica will be removed from the actual chrome ore used in accordance with the present invention.

The following typical analyses are given for Phillipine chrome ore before and after this treatment to remove the silicate phase:

The mol ratio of A1203 plus CI'zOs to MgO in a typical purified spinel of one of the currently used chrome ores employed in making refractory brick is about 1.35 to 1. Such an ore is considered satisfactory for making refractory brick because of the relationship of these principal refractory oxides. The remaining oxides of iron may not all be present as FeO as indicated in the above analyses, but the iron may be considered as existing principally as an R oxide, the ratio of total R203 to R0 being approxi-- mately unity. Such a mol ratio of A1203 plus Cr203 to MgO is typical of all the chrome ores which are considered acceptable for refractory brick manufacture.

Another type of chrome ore exists, however, which is characterized by a much higher ratio of its refractory oxides A1203 plus CrzOz to MgO. A typical ore of this character is mined in the Transvaal district of South Africa, and is available on the market in the chemical industry as Transvaal chrome ore. It has not heretofore been possible to produce satisfactory refractory brick from chrome ores of the Transvaal type. Transvaal chrome ore has the following typical chemical analyses before and after treatment with sulphuric acid plus hydrofluoric acid and finally leaching with dilute hydrochloric acid to remove silica:

The mol ratio of A1203 plus CrzOs to MgO in the spinel phase of the Transvaal chrome ore purified to remove silica is about 2.07 to l.

The iron oxide in the Transvaal type of chrome ore exists principally as FeO (although some may be present as F6203) and the FeO acts to restore the ratio of R203 to R0 to approximately unity. Unfortunately, however, FeO is not a stable refractory oxide. Upon heating Transvaal chrome ore FeO oxidizes to F6203 and the balance of R203 to R0 in the spinel shifts, with disastrous results to the properties of the refractory. Upon heating Transvaal chrome ore to about M00" 0. the effect of such oxidation is evidenced by the appearance of F6203 under petrographic examination. Furthermore the X-ray diffraction of the purified chrome ore before heating shows a definite spinel pattern with no significant unidentified lines. After heating, however, while the spinel lines still remain, numerous other lines appear indicating change in the oxide balance in the crystal of the spinel from Transvaal chrome ore.

Thus the chrome spinel from such a chrome ore having a deficiency of refractory MgO in the R0 portion so that the molar ratio of A1203 plus CizOx to MgO exceeds 1.75 is not acceptable for the manufacture of refractory brick. In fact Transvaal chrome ore is not used for such purposes.

An extensive study has been made by me of the behavior of chrome ore of the Transvaal type at high temperatures, and I have found as a result of such study that upon heating such ores in the presence of magnesia to temperatures of about 1300 to 1400 C. a remarkable change occurs. The magnesia is actually absorbed by the crystalline spinel phase, which changes its character. For example, heating 75 percent of purified Transvaal chrome ore with 25 percent of magnesia to 1400" C. for 5 hours and then treating the product with a 50 percent solution of hydrochloric acid to remove magnesia or other constituents foreign to the spinel phase, the spinel phase has been found to have increased in magnesia content to about 24 to 25 percent, and the molar ratio of A1203 plus C12O3 to MgO has become less than unity. By this action the possible damage done by FeO is minimized and a refractory phase is formed in which the balanced ratio of the R203 to R0 is achieved by stable refractory oxides, while the shifting of the iron by oxidation from FeO to F8203 is counteracted by the magnesia added. The X-ray diffraction reveals a definite spinel pattern in the higher magnesia crystal.

Thus by the above described action of magnesia I am able to make Transvaal chrome ores available for refractory brick manufacture, whereas they have heretofore been considered unsuitable.

I have discovered that this method of making Transvaal chrome ore suitable for refractory brick manufacture is particularly desirable in the manufacture of refractory brick which are suited for use without kiln firing. These are sold as so-called unfired brick. In the processing of brick suitable for use without kiln firing, one of the principal difficulties is the linear shrinkage which occurs when the unfired brick are subjected to the high furnace temperatures during use. I have discovered that bricks based on Transvaal chrome ore to which magnesia has been added to correct the ratio of R203 to R0 are free from such shrinkage. At high temperature the FeO is oxidized and the MgO enters the chrome crystal, thereby causing it to expand somewhat, and this overcomes the tendency to shrink otherwise present in such unfired brick. The net result is a brick with substantially no natural shrinkage mach ne tionsrofithepresent inventi'om are employed; "The presence of silicaimpurities is-not objectionable i'an'dt I do not: find it: desirable to I remove sil'i'cia byrchemicalimeansdn making commercial brick; as

I did'fihi studyingthe behavior: of the spinelphase. The-chromeore is. crushed and-'screened toform coarse particles which pass-ea 'Tyl'en standard screen-of about fi 'mesh per-linear inch-and rest on: a. screen: of about 28 mesh per linear inch. Icprefer-to; use; 65 percentof such coarse chrome particles with. 35: percent of magnesia: fine particles groundito pa'ss a screerr of at -l'east Tyler standard: 48 mesh. per linear inch;

The=. magnesia used? should: be d'ead burned natural zmagnesite :or: peri'clase prepared by calcining magnesium hydrate (prepared from sea -.vvater-or:brine-) at temperatures-of about '1600 *'C. -A; typical anai'ysis of such seawater magnesia Per cent Ign. loss- 0.0 $102 3'10 Eat) 3L4 MgOtbydiff; 9013 I. add moisture anda. bonding .agent such. as sulphuric. acid, sodium, acid sulphate. magnesium sulphate, magnesiumchl'oride, sulphiteliquor or other organicl bondingi agent... The quantity of bond used is ordinarily of the order of 1 percent but in any caseiwillnot-exceedefi percent of the brick, after removal of the water by drying. The tempered brick mixture is pressed into brick form using a molding pressure desirably in excess of 5,000 p. s. 1., preferably 10,000 p. s. i. or more. The pressed brick are dried to remove moisture and develop the bond.

If desired the brick may be treated with carbon dioxide gas under pressure preliminary to drying to improve the bond. This is described in my U. S. Patent No. 2,547,323, granted April 3, 1951, for Unburned Refractory Brick Making.

For certain uses I may comold the refractory brick with steel sheets pressed on the principal faces in accordance with my U. S. Patents Nos. 2,247,376 and 2,289,911.

The brick produced according to the present invention are useful for all high temperature furnaces where chrome-magnesite brick are employed, as for example open hearth steel furnaces, copper smelting furnaces, and other high temperature melting, smelting and refining furnaces.

The preferred composition of the brick is 65 percent of coarse Transvaal chrome ore partiof line particles by employing- 20 percent of fine magnesiaparticl'es and l5-percent of fine chrome particles;

On the other hand;the amount of'lfine magnesia particles can' be' increased to" 40 percent or evenn-50 percent of the" brick, correspondingly reducingthe" quantity of" coarse particles to '60 percent or even 50 percent (the coarse particles being ordinarilycoarse Transvaal chromite) The'coarse-chromei ore" may be replacedL-by coarse magnesia passing through 6' or preferably 8- Tylerstandard meshper linear inch, and resting oir a 28 mesh: screen, retaining as li'ttle as 20=-percent of' coarse Transvaalchromite par,- ticles and using coarse magnesia particlesto in crease the quantity of coarse; particlesto for exa-mple' 6 5 percent;

Thus it will be seenthatthequantity of Transvaal chromitetmay vary between percent and 20 percent of the brick andthe quantityofmagnesia may-"varybetween- 20 percent and 80=percentof the brick; while the-quantity of coarse particles may varybetween 50- and -80 percent of thebrick (the coarse-particles beingeither Transvaal chromite or Transvaal chromite and magnesiawithin tha-limits set forth) and the quantity of fine particles may vary between 20 and-50 percent of the brick (the fine" particles being magnesia or a "mixture of Transvaal chromite and magnesia within the limits-set) Inthe cases-of the percentages of the composition of the brick; the percentages are by Weight, based upon therefractorymaterial present-ignoring the bond; which in any'case of the-order of I'ypercent and not exceeding 5 percent of the-brick;

The procedure-used in the present invention produces a brick of increased strength orhigh modul'usof rupture'without; burning and suitable for use in: theunfired" condition. For example, a i brickcontaining 65- percentof Transvaal chrome ore and 35 percent of sea water magnesia on reheating to 1650 C. for five hours showed no permanent shrinkage but had a desirable linear expansion of about 1 percent. This brick compares very favorably with those heretofore made fro mthe so-called refractory chrome ore, such as Cuban, Philippine, or Turkish chrome ore, which for making refractory brick where Transvaal chrome ore has theretofore been supposed to be unsuitable.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art to obtain all or part of the benefits of my invention without copying the process and product shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire by Letters Patent is:

1. The process of stabilizing chrome ore having a mol ratio of A1203 plus C12O3 to MgO exceeding 1.75 for refractory brick purposes, which comprises mixing chrome ore having a mol ratio 7 of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed) with magnesia in the proportions of 20 to 80 percent by weight of magnesia, and forming the mixture into a refractory brick.

2. The process of stabilizing chrome refractory made from chrome ore having a mol ratio of A1203 plus Cr20a to MgO exceeding 1.75, which comprises mixing with chrome ore having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed), from 20 to 80 percent by weight of magnesia, forming the mixture into a refractory brick without kiln firing and subjecting the refractory brick to firing temperature for the first time in the furnace during use.

3. The process of stabilizing chrome ore having a mol ratio of A1203 plus 01*203 to MgO exceeding 1.75 for brick manufacture, which comprises mixing together from 80 to 20 percent by weight of chrome ore having a mol ratio of A1203 plus Cr2O3 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed) and from 20 to 80 percent by weight of magnesia, the mixture including 50 to 80 percent by weight of coarse particles between 6 and 28 mesh per linear inch and 20 to 50 percent by weight of fine particles through 48 mesh per linear inch, from 20 to 50 percent by weight of the mixture being magnesia fine particles and incorporating the mixture into a refractory brick ready for use without kiln firing.

4. The process of making a stable refractory brick from chrome ore having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75, which comprises mixing about 65 percent by weight of coarse particles of chrome ore having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed), between 6 and 28 mesh per linear inch and about 35 percent by weight of magnesia fine particles through 48 mesh per linear inch, and incorporating the mixture into a refractory brick ready for use without kiln firing.

5. A non-acid refractory brick essentially consisting of from 80 to 20 percent by weight of chrome ore having a mol ratio of A1203 plus CraOs to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed) and from 20 to 80 percent by weight of magnesia.

6. A non-acid refractory brick essentially consisting of from 80 to 20 percent by weight of chrome ore having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination being made from a sample from which silica was removed) and from 20 to 80 percent by Weight of magnesia made up from 50 to 80 percent by weight of coarse particles between 6 and 28 mesh per linear inch and from 20 to 50 percent by weight of fine particles through 48 mesh per linear inch.

7. An unfired non-acid refractory brick suitable for use in unfired condition essentially consisting of from 80 to 20 percent of chrome ore and consisting of from 50 to 80 percent of coarse particles between 6 and 28 mesh per linear inch and from 20 to 50 percent by weight of fine particles through 48 mesh per linear inch, at least 40 percent by weight of the particles of chrome ore having a mol ratio of A1203 plus Cr2O3 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed) and the balance being essentially magnesia, at least 20 percent by weight of the brick being magnesia fine particles.

8. An unfired non-acid brick suitable for use in unfired condition essentially consisting of 80 to 20 percent by weight of coarse particles of chrome are having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed), ranging in size between 6 and 28 mesh per linear inch and between 20 and 50 percent by weight of magnesia particles through 48 mesh per linear inch.

9. An unfired non-acid refractory brick essentially consisting of about percent by weight of coarse particles of chrome ore having a mol ratio of A1203 plus Cr203 to MgO exceeding 1.75 (the mol ratio determination having been made on a sample from which silica was removed) and ranging in size from 6 to 28 mesh per linear inch and about 35 percent by weight of magnesia particles through 48 mesh per linear inch.

RUSSELL PEARCE HEUER.

No references cited. 

1. THE PROCESS OF STABILIZING CHROME ORE HAVING A MOL RATIO OF AL2O3 PLUS CR2O3 TO MGO EXCEEDING 1.75 FOR REFRACTORY BRICK PURPOSES, WHICH COMPRISES MIXING CHROME ORE HAVING A MOL RATIO OF AL2O3 PLUS CR2O3 TO MGO EXCEEDING 1.75 (THE MOL RATIO DETERMINATION HAVING BEEN MADE ON A SAMPLE FROM WHICH SILICA WAS REMOVED) WITH MAGNESIA IN THE PROPORTIONS OF 20 TO 80 PERCENT BY WEIGHT OF MAGNESIA, AND FORMING THE MIXTURE INTO A REFRACTORY BRICK. 